The State Department has yet to comment on the deal, though the U.S., Russia, and France had clearly endorsed the initial agreement reached in Vienna that would have Iran swap the majority of its low-enriched uranium for more highly- enriched nuclear fuel rods, a form more difficult to use in developing a nuclear weapon.

Iranian media reports, supported by European diplomats close to the negotiations, had suggested Iran wants to ship its uranium in instalments, rather than exporting the bulk of its stockpile for enrichment in Russia.

Analysts say Iran currently has a known sum of 1,500 kilograms of enriched uranium, in the form of uranium hexafluoride. About 1,000 kilograms would be needed to develop a nuclear weapon.

The proposed nuclear fuel swap, publicly advanced at talks in Geneva and then refined in Vienna, would have 1,200 kilograms – roughly 80 percent of Iran's known stockpile – sent to Russia. France and the U.S. have insisted that the amount be shipped in one installment, while Iran has suggested it wants to send the fuel abroad in stages.

"It's a deal breaker," said Jacqueline Shire of the Institute for Science and International Security in Washington. "It's no dice if they want to do an instalment plan."

"The bottom line of the Vienna was this: suspension of sanctions and war and tensions for a year, for suspension of Iran's ability to make a bomb," said Meir Javedanfar, a Middle East analyst based in Israel. "If this is Iran's official answer it's going to set back negotiations."

A second round of talks in Geneva between Iran and the P5+1 powers, originally scheduled for this week, hinges on the success of the proposed nuclear deal. A diplomat close to negotiations said future talks were "not on the horizon," stating that the lack of agreement on the proposed deal held back any possible agenda for another meet.

"The situation remains pretty pretty grave. It's clear Iran wants to talk, but that's not enough," said the diplomat, who spoke on a non-attribution basis given the sensitivity of the talks.